That's a small positive step that you can boot from an external. But there's an important distinction to be made. To test the MBP disk vs the rest of the components you need to connect the external to the MBP and then install OSX. I.e. you are using the MBP the perform the installation, not the iMac as you are at the moment.

Use liondiskmaker on your iMac to create an installer DVD then use this and the external on your MBP and perform an OSX install to the external.
If this install works and subsequently boots to the external ok then it definitely points to your internal hard disk at fault (a straightforward replacement you could do yourself).
If the install still fails then it's very likely a non-user serviceable part that's at fault

External hard disk acquisition addict - but admitting the problem is the first step to a robust backupPlease use the reputation system if you think you've been helped - top right of this post